Subframe off. This is another thing where practice helps.
And, as advised earlier, I duly went nuts with the welder, then the grinder (shh - some of my welds were a bit on the ugly side. Funny how the tighter the spot, the uglier the weld.)
Then some pressure-pak "Rust Reformer", and -

One (1) Acme No.1 Body Crossmember.
Then I centre-punched the spot-welds along the RH sill bottom, drilled them, and got out with the Spot-Weld Separator (patent applied for).

Don't you just love a good clear-vision sill?
I had a stroke of luck in the sheet-metal shop on Friday. In the offcuts bin was a narrow strip of 1.6 sheet, and it was just the size I needed. So the sheet-metal worker bent it to the right angle (not
a right angle though). Here it is, in place on the inner sill. I need to slot and weld it for the drain slots (marked with chalk).

And then I decided that I'd had enough for one weekend and went inside.
Rob